This invention pertains to frequency synthesizers and phase locked loops and, more particularly, to a battery saver circuit for a frequency synthesizer that incorporates a selectable automatic frequency control circuit.
The introduction of frequency synthesizers into portable, battery operated radios has, to a considerable extent, increased the number of channels that these radios can receive or transmit on. The disadvantage of a synthesizer, however, is that it consumes considerable more current than a conventional crystal controlled oscillator circuit.
One of the primary design objectives of a portable, battery operated radio is to achieve the maximum time between battery charges or replacements, i.e., to achieve the minimum current drain on the battery. Accordingly, the invention described below is a frequency synthesizer circuit that substantially reduces the current drain of its battery power source, thereby prolonging battery life.
The primary application for this invention is in trunked radio communication systems. Trunked radio systems include a plurality of "voice" channels and at least one control channel. The control channel is a duplex channel, i.e., it includes an outbound (base station to remote unit) frequency, and an inbound (remote to base) frequency. A radio base station transmits a continuous signal on the outbound control channel frequency. When a remote unit (e.g., a portable or mobile radio transceiver) operator initiates a call to another remote unit, a request is transmitted on the inbound control channel frequency for a voice channel assignment. If a voice channel is available, the base station transmits an assignment of a particular channel to the requesting remote unit by appropriately modulating the request on the continuously transmitted outbound control channel signal. Upon receiving this assignment, the requesting remote unit switches to the assigned voice channel. When the call is completed, the remote unit switches back to and continuously monitors the outbound control channel frequency until directed to go to another channel by the base station.
The invention described below takes advantage of the continuous transmission of the control channel signal by the base station. This invention employs an automatic frequency control circuit that frequency locks the synthesizer to the control channel signal whenever the remote radio unit is receiving on the control channel. When frequency locked in this manner, the invention turns off one or more of the circuits in the frequency synthesizer, thereby reducing battery drain.